When a device such as a personal computer, fax machine, etc is connected to a telephone line, a modem is required to receive an incoming analog signal from the telephone line and convert the signal to a digital signal for use by the computer. Conversely, the modem receives outgoing digital signals from the computer and converts them into an analog signal for transmission over the telephone line.
An "online" modem is engaged electrically in parallel with the telephone line when a single telephone line is used. Therefore, a calling party attempting to place a phone call will receive a busy signal if the computer is in use, and the computer user will be unaware of the attempted call. The calling party must guess when the user is not on the computer, fax machine, etc. in order to place an incoming phone call, which may take many attempts, thereby increasing the traffic on the telephone network. Furthermore, important calls may go undetected by the computer or fax machine user.
A call waiting feature is offered by many telephone companies. This feature alerts a person who is in the middle of an ongoing telephone call that another party is attempting to contact them by telephone. A typical call waiting signal is generally made up of a first call waiting signal at a predetermined frequency (e.g., 440 hertz) for a predetermined duration (e.g., 0.5 seconds) followed by a delay period (e.g., 10 seconds) followed by a second call waiting signal similar to the first call waiting signal. The call waiting signal is embedded in various other signals along the telephone line, however, and is not easily discernible. Further, when a computer or fax modem is connected and operational, there are no means to hear the call waiting signal.
Though the call waiting feature works well between telephone users, the feature is incompatible when applied to the computer or fax machine user. There have been several attempts to design a modem that can detect a call waiting signal. Modems attempting to incorporate such a feature to date, however, are not reliable. In addition, modems having call waiting detection capability are expensive and reduce the transforming capability of the modem.
U.S. Pat. No. 5,287,401 to Lin, for example, discloses a modem that detects a call waiting signal from a telephone line. The modem attempts to monitor the presence of the call waiting frequency. Upon detection, the modem then monitors the signal for what would be the typical duration of the call waiting signal. The signal is subsequently monitored for the absence of a call waiting signal (e.g., the delay period) and then over a period equivalent to the second call waiting signal. This detection technique has several problems. First, the technique is not reliable, because a random 440 hertz signal could be generated amid the rapidly changing analog modem signals as data is being transferred. Second, this monitoring technique takes a longer time than the call waiting signal is on the line, due to the time period used for monitoring for the absence of a call waiting signal (e.g., the delay period). Moreover, the modem must transfer its function to monitor for a frequency rather than transform data, which wastes modem capability. Finally, by the time a modem detects the call waiting signal and the user logs off the modem connection, the user may miss the call anyway.